[snapshot] support multiple contexts in the same snapshot.

src/debug/debug.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/debug/debug.h"
 
 #include "src/api.h"
 #include "src/arguments.h"
 #include "src/bootstrapper.h"
 #include "src/code-stubs.h"
@@ skipping 519 matching lines @@
   SuppressDebug while_loading(this);
 
   // Disable breakpoints and interrupts while compiling and running the
   // debugger scripts including the context creation code.
   DisableBreak disable(this, true);
   PostponeInterruptsScope postpone(isolate_);
 
   // Create the debugger context.
   HandleScope scope(isolate_);
   ExtensionConfiguration no_extensions;
+  // TODO(yangguo): we rely on the fact that first context snapshot is usable
+  //                as debug context. This dependency is gone once we remove
+  //                debug context completely.
+  static const int kFirstContextSnapshotIndex = 0;
   Handle<Context> context = isolate_->bootstrapper()->CreateEnvironment(
       MaybeHandle<JSGlobalProxy>(), v8::Local<ObjectTemplate>(), &no_extensions,
-      DEBUG_CONTEXT);
+      kFirstContextSnapshotIndex, DEBUG_CONTEXT);
 
   // Fail if no context could be created.
   if (context.is_null()) return false;
 
   debug_context_ = Handle<Context>::cast(
       isolate_->global_handles()->Create(*context));
 
   feature_tracker()->Track(DebugFeatureTracker::kActive);
 
   return true;
@@ skipping 2057 matching lines @@
 }
 
 
 void LockingCommandMessageQueue::Clear() {
   base::LockGuard<base::Mutex> lock_guard(&mutex_);
   queue_.Clear();
 }
 
 }  // namespace internal
 }  // namespace v8